Islamic State of Iraq, the country's al Qaeda wing, is regaining strength, invigorated by the Sunni Muslim rebellion in next door Syria and has carried out dozens of high-profile attacks since the start of the year.

"What has reached you on Tuesday is just the first drop of rain, and a first phase, for by God's will after this we will have our revenge," the al Qaeda statement posted on a jihadist website said.

Car bombs and suicide blasts hit mainly Shiite districts in Baghdad and other cities on Tuesday.

Suicide attackers have struck nearly two times a week since January, a rate Iraq has not seen for several years.

Sunni Islamists see Iraq's Shiite-led government as oppressors of the country's Sunni minority and target Shiites to try to provoke a sectarian confrontation like the inter-communal slaughter that killed thousands in 2006-7.

A decade after U.S. and Western troops swept into Iraq to oust Saddam Hussein, the oil-producing country still struggles with sectarian tensions and political instability that test the fragile unity among Shiite, Sunni and Kurdish ethnic groups.

Ten years after the US launched a "shock and awe" campaign toppling Saddam Hussein, the cost of the Iraq War is now estimated to be about $2 trillion -- but the region is far from stable. NBC's Richard Engel reports.